Photosensitive resins used for the manufacture of relief printing plates are preferably developable with water rather than organic solvent for various reasons such as ease of handling, health of workers who come in contact therewith, safety, and avoidance of environmental pollution. Printing plates employed for flexographic printing must be capable of printing on a variety of substrates, which vary widely in their composition and surface uniformity. Substrates employed for flexographic printing include metal foils, plastic films, kraft paper, corrugated board, laminated papers and boards, newsprint, and the like. In order to successfully print on this wide variety of substrates, one must use a variety of ink types and printing press conditions.
The ideal printing plate for use in flexographic printing applications would be one which is capable of withstanding the potentially deleterious effects of the agents which are likely to come in contact therewith, i.e., common ink solvents, developing media (e.g., water), and radiation-cured materials. The ideal printing plate will further show no tendency to crack when flexed, and will be soft enough to conform to irregular surfaces, yet durable enough to withstand the forces of the printing process. It is also desirable that the resin material employed for the preparation of printing plates resist cracking or degradation when exposed to ozone in ambient air.
Another desirable feature of resin materials employed for the production of printing plates is a sufficient level of sensitivity and reactivity so as to allow digital imaging thereof. Digital imaging is expanding rapidly in all areas of the graphic arts. Ongoing advances in cameras, computers, networks and software are making it increasingly easy to capture, transmit, manipulate and render images composed entirely of electronic bits. The use of completely digital pre-press processes is becoming more and more commonplace in many areas of printing. A key step in digital pre-press processes is computer pagination, where all of the components of a page image are "pasted-up" electronically while displayed on a computer monitor.
Once paginated, the logical next step is to produce printing plates directly from the digital page data. Devices that enable this direct production of printing plates are commonly called platesetters, which are an essential component of computer-to-plate (CTP) technology. No film is required and there are no intermediate renditions of the image between the original art (text, photos, ads, and the like) and the printing plate. The pre-press page flow is, therefore, reduced by one or more steps.
Flexographic printers realize a number of advantages upon adoption of digital technology, in general, and digital platemaking, in particular. Firstly, print quality is improved. This includes better vignetting, tighter registration, sharper line work, and more open shadows/midtones. On-press registration is also better, which adds to the quality improvement for process color as well as reduces startup paper waste--a significant advantage with higher cost papers.
The elimination of film is another important goal for flexographic printers, since film costs can be $0.40-$0.75 or more per square foot (including chemicals). Other film related costs include the purchase and maintenance of capital equipment (page cameras, imagesetters, processors and punch units), manpower and waste treatment. The manpower requirements of CTP plate production are reduced because there is no longer a need to handle, inspect and opaque film. The treatment of heavy metal (silver) waste is also eliminated. Indeed, computer-to-plate technology totally obviates this environmental concern and its associated costs.
Platesetters for offset plates have been commercially available for a number of years. Because of the thinness of offset coating (1-2 microns), the technology to image offset plates is less demanding than that needed for flexo's thick relief (&gt;355 microns). The offset technology is now field proven and offered by more than 20 vendors in a myriad of configurations. For example, one can choose from blue, green or infra-red lasers in either external drum, internal drum or flatbed formats.
In flexography, there are currently two commercially available technologies, i.e., laser ablation and mask ablation. In laser ablation, a high power carbon dioxide laser, controlled by a computer, blasts away the non-image area of a solid rubber plate. The imaging speed is only about 1 m.sup.2 per hour, however, which is too slow for many applications. The laser ablation approach also lacks the ability to resolve the detail of fine halftone screens.
In mask ablation, a photopolymer is manufactured with a thin, opaque layer on top. The opaque layer is then ablated away in the image area by an IR laser to create a mask directly on the surface. The underlying photopolymer is then exposed through the mask with conventional UV lamps, and solvent processed. Imaging employing this technique is slow, however (.about.1 m.sup.2 /h), and the mask technology adds significantly to the cost of the printing plate.
What is needed in the field of flexographic printing is high sensitivity photopolymer capable of direct exposure by a rapidly moving laser beam to produce plates of high quality. Currently, there are no commercially available flexographic resins which are capable of rapid, direct exposure by lasers.
Several types of photosensitive resin formulations have been developed for use in flexographic printing. Photosensitive resin plates prepared employing modified thermoplastic elastomeric rubber or rubber-like printing media have excellent mechanical properties, but can be processed only in organic media. See, for example, U.S. Pat. Nos. 4,323,637, 4,369,246 and 4,622,088. None of the photosensitive resin formulations described in the art are sufficiently photoreactive, however, to allow digital imaging thereof.
More recently, solid flexographic printing plates which are developable in water have been described. See, for example, U.S. Pat. No. 5,348,844 (which describes photosensitive printing media having a composite structure comprising discrete domains of water-dispersible latex copolymer and a photopolymerizable interstitial phase) and U.S. Pat. No. 5,073,477 (which describes photosensitive resins comprising a latex copolymer component, a liquid or solid copolymer component, a basic nitrogen atom-containing compound, an ethylenically unsaturated compound and a photoinitiator). The solid photosensitive resin formulations described in '844 and '477 are not sufficiently photoreactive, however, to allow digital imaging thereof.
In summary, each of the above-described systems suffer from certain drawbacks which limit the usefulness thereof, e.g., the resulting resins undergo excessive swell in aqueous inks (which results in poor image reproduction), the resulting resins lack sufficient structural integrity to withstand the physical rigors of flexographic printing operations, the resulting resins produce materials which are excessively hard or soft (and hence do not meet the performance requirements of flexo users), the resulting resins still require the use of washout aids (e.g., acid, alkali, surfactant, etc.), in the aqueous media employed for developing the resulting printing plate, the resulting resins lack sufficient storage stability to allow long-term storage of printing plates prior to photopolymerization thereof (i.e., washout of unexposed material becomes more difficult upon extended storage) and/or the resulting resins are not sufficiently reactive to allow digital imaging thereof. Accordingly, what is still needed in the art are resins useful for the production of flexographic printing plates which have improved photosensitivity and physical properties.